1. Field of the Invention
The present invention relates to a heat switch and, more particularly, to a heat switch used in conjunction with a cryogenic refrigerator.
2. Description of Related Art
A heat switch is used to conduct heat when closed, and to prevent heat conduction when open. It is used in a variety of cryogenic application where temperature must be controlled and, in particular, comprises an essential component of an adiabatic demagnetization refrigerator (xe2x80x9cADRxe2x80x9d). ADRs are discussed in C. Hagmann and P. L. Richards, xe2x80x9cAdiabatic demagnetization refrigerators for small laboratory experiments and space astronomy,xe2x80x9d Cryogenics, vol. 35, no. 5, 1995, pp. 303-309. As noted therein, ADRs are capable of reaching operating temperatures below 0.01 K, but typically operate at temperatures near 0.1 K. Such refrigerators are commonly used for small laboratory experiments, space astronomy, and detectors for millimeter waves, X-rays and dark matter. ADRs can also operate in zero gravity, which would make them useful in satellites and space vehicles.
An ADR typically includes a paramagnetic material suspended in the refrigerator, e. g., a paramagnetic salt such as ferric ammonium alum or chronic cesium alum. The paramagnetic material is in thermal contact with an elongated metal rod called a xe2x80x9ccold finger,xe2x80x9d which is, in turn, in thermal contact with a cold stage. The cryogenic experiment or instrument that makes use of the cold provided by the ADR is attached to the cold stage. When closed, the heat switch provides for thermal conduction between the paramagnetic material and a heat sink having a temperature of 1xc2x0 to 4xc2x0 Kelvin.
The ADR cycle begins by closing the heat switch to thermally connect the paramagnetic material to the heat sink. A strong magnetic field is then applied to the paramagnetic material to align the magnetic moments of the material. This reduces the entropy of the moments, and the heat of magnetization thereby released is transferred by conduction through the closed heat switch to the heat sink. This process is isothermal.
The switch is then opened to thermally isolate the paramagnetic material from the heat sink as well as extraneous sources of thermal energy, and the applied magnetic field is decreased. The temperature of the material decreases as magnetic moments in the material lose their alignment and entropy is transferred from the lattice to the magnetic moments. The result is an adiabatic drop in the temperature of the paramagnetic material and thus the cold stage. When the material is partially demagnetized to a desired operating temperature, the temperature can be held constant by very slowly reducing the magnetic field to compensate for heat leakage. By regulation of the magnetic field, a stable temperature can be maintained in the cold stage for many hours, after which the cycle is begun again.
The requirements for heat switches to be used in conjunction with ADRs include a high ratio of dosed to open thermal conductivity, reliability, and low heat emission from the operation of the switch itself. Mechanical, electro-mechanical, and gas-gap heat switches have all been used. Gas-gap heat switches use the thermal conductivity of helium contained between two surfaces. Activated charcoal absorbs the He gas when the switch is open. Because they are always closed at a temperature less than 30xc2x0 Kelvin, the initial cool-down of the ADR is facilitated. However, this type of switch has the disadvantage of having a finite conductance in the open state due to conduction through the gas tight container. Furthermore, the charcoal must be warmed to absorb the He gas, and this comprises the dominant heat leak for small ADRs. As a result of the two foregoing drawbacks, the mass of paramagnetic salt must be increased in order to obtain a useful cold period, causing concomitant increases in the weight and size of the ADR. C. Hagmann and P. L. Richards, supra, at 306.
In view of the aforementioned problems endemic to gas-gap heat switches, it has been found that mechanical and electromechanical heat switches offer the best performance. An electromechanical heat switch is described and illustrated in C. Hagmann and P. L. Richards, supra, at 305. The heat switch shown therein uses a solenoid to force the translation of a plunger that, in turn, forces jaws into normal contact with a cold finger. The drawback attendant to this apparatus is that to keep the jaws closed and in continuous contact with the cold finger, the solenoid must remain actuated for the duration of the isothermal heat transfer to the heat sink, as well as during the lengthy period required to initially cool the ADR down from room temperature. The continuous current generates heat from ohmic resistance that will be conducted into the heat sink and the ADR, thus adversely affecting the ADR""s performance. Furthermore, this continuous actuation reduces the force the solenoid can generate and apply below that available during a short actuation pulse.
There is a need in the art for an electromechanical heat switch that remains closed with sufficient normal force on the cold finger to provide thermal conduction to the heat sink, but without generating the increased thermal energy attendant to keeping the solenoid operative throughout this step of the refrigeration cycle. The present invention not only fulfills this need in the art, but also applies a normal force against the cold finger greater than that of the heat switches of the prior art, and thus improves on the thermal conduction provided by the prior art switches.
Briefly, the present invention is a heat switch this is of particular advantage when used in conjunction with an ADR. The heat switch includes two symmetric jaws. Each jaw is comprised of a link connected at a joint to a flexible arm. The joints are translatable. Each arm rotates about a fixed pivot, and has an articulated end including a thermal contact pad that is thermally connected to a cryogenic heat sink by a metal braid. The links are joined together at a main joint that can move along a path that is collinear with the longitudinal axis of the plunger for a closing solenoid.
To close the switch, the closing solenoid is actuated and its plunger forces the main joint to an over-center position, beyond an unstable center position where the two links are aligned with their respective arms at the joints. This movement rotates the arms, bends them into a stressed configuration, and forces the thermal contact pads towards each other and into compressive contact with a cold finger that lies between them. This contact provides for the exhaust to the heat sink of the heat of magnetization released during the application of a magnetic field to the paramagnetic material inside the ADR. It also provides thermal contact for cooling the paramagnetic material and cold stage when the heat switch is closed during the initial cooling from room temperature.
Once the over-center position of the main joint is achieved, the closing solenoid is deactivated. The heat switch remains closed by virtue of the restoring force applied by the stressed arms. When the isothermal heat exhaust step is completed, actuation of an opening solenoid opens the heat switch by pushing the main joint up and beyond the center position, and the heat switch is returned to its starting open-switch position. With the switch open, the cold stage is thermally isolated from everything except the paramagnetic material. The cold stage is cooled by gradually decreasing the applied magnetic field.